Import capturer fails to error cleanly on submodule imports without 'asname' values.

[DavidCEllis]

Currently in the tracing back, a submodule import without an assigned name will trigger a keyerror as it fails to be traced backwards.

from ducktools.lazyimporter import LazyImporter
from ducktools.lazyimporter.capture import capture_imports

laz = LazyImporter()

with capture_imports(laz, auto_export=False):
    import unittest.mock

Errors as:

Traceback (most recent call last):
  File "C:\Users\ducks\Source\ducktools-lazyimporter\scratch\example.py", line 6, in <module>
    with capture_imports(laz, auto_export=False):
         ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\ducks\Source\ducktools-lazyimporter\src\ducktools\lazyimporter\capture.py", line 236, in __exit__
    capture = importer_map[attrib_name]
              ~~~~~~~~~~~~^^^^^^^^^^^^^
KeyError: 'unittest'

This is because the name in the global namespace isn't the submodule, it's the initial parent import.
The importer map is constructed based on the final element, so an "as" import can work backwards to the original import.

If the imports are reversed, there is currently no error - but the submodule doesn't work as the module import overwrote the submodule import in the globals namespace.